Gasification is a process that converts organic or fossil based carbonaceous materials into carbon monoxide, hydrogen, carbon dioxide and methane. This is achieved by reacting the material at high temperatures without combustion, with a controlled amount of oxygen and/or steam, to generate a resulting gas mixture of H2 and CO called syngas. Syngas is combustible and often used as a fuel or as an intermediate for the production of other chemicals such as methane, methanol, synthetic diesel and dimethyl ether in catalytic processes.
A significant use of syngas is as a fuel for fuel cells, which utilize hydrogen and oxygen to produce direct current electricity. In addition to electricity, fuel cells produce water, carbon dioxide, heat and, depending on the fuel source, very small amounts of nitrogen dioxide and other emissions. The energy efficiency of a fuel cell is generally between 40-60%, or up to 85% efficient if waste heat is captured for use. Additionally, because fuel cells generate electricity while keeping the fuel and air separated, the CO2 generated in the anode of the fuel cell is not diluted with nitrogen, making ˜100% capture of CO2 within the system both technically and economically viable. Fuel cells therefore hold great promise in efforts toward reducing CO2 emissions, even when the fuel source is coal.
A Solid Oxide Fuel Cell (SOFC) is a specific type of fuel cell which offers particular advantage. SOFCs use a solid material, typically yttria-stabilized zirconia, as the electrolyte, and the solid material construction allows geometries outside of the flat plane configurations of other types of fuel cells. The SOFC operates at very high temperatures, typically between 500 and 1000 degrees Celsius (° C.), and are capable of internally reforming light hydrocarbons such as methane, propane and butane. As a result, SOFCs can be run on a variety of fuels other than pure hydrogen gas, provided the fuel selected contains hydrogen atoms. The high SOFC temperatures also incur other advantages, such as: a) the ability to incorporate bottoming cycles to generate further power from high temperature exhaust stream, b) the capability to water-gas-shift CO into H2 fuel, c) the capability to steam reform hydrocarbons into H2 and CO, and d) the capability to catalyze the electrochemical reactions using non-noble metals, thus eliminating the need for expensive electro-catalysts, such as platinum. A primary disadvantage in SOFC operation is the necessary cooling load of the fuel cell due to the exothermic electro-chemical reaction between hydrogen and oxygen ions. In terms of overall system efficiency, the parasitic cooling load typically manifests as compressive and pumping power cost expended for supply of cooling air flow. A reduction in cooling requirements therefore has direct improvement on the operating efficiency of the SOFC.
One method of reducing SOFC cooling requirements is to use the internal reforming capabilities of the electrocatalysts engendered by the high operating temperatures. Endothermic reforming of hydrocarbons such as CH4 can serve as an effective heat sink to the exothermic H2 oxidation within the fuel cell, and significantly reduce stack thermal management load. As a result, use of a methane-rich syngas as an SOFC fuel carries distinct advantages. However, generation of a methane-rich syngas through gasification has been problematic at the typical SOFC operating temperatures, temperatures above 700° C., because of rapid methane reforming and oxidation in typical coal gasifiers. Correspondingly, the realizable efficiency of combined cycle concepts such as the Integrated Gasification Fuel Cell (IGFC) have suffered. In order to achieve high IGFC efficiencies, it is desirable that the gasification process and the SOFC operation occur at commiserate temperatures, and additionally desirable that the gasification process delivers a methane-rich syngas product, in order to exploit the endothermic reforming reactions discussed above that minimize the amount of coolant or cooling air. See Siefert et al., “Integrating Catalytic Coal Gasifiers With Solid Oxide Fuel Cells,” ASME 2010 8th International Fuel Cell Science, Engineering and Technology Conference: Volume 2 (2010); see also Romano et al., “Integrating IT-SOFC and gasification combined cycle with methanation reactor and hydrogen firing for near zero-emission power generation from coal,” Energy Procedia 4 (2011), among others. A gasification system capable of generating a methane-rich syngas at the high operating temperatures of an SOFC would provide distinct advantage. The advantage of a methane-rich syngas would additionally accrue to the gasification operation itself, since methanation is a highly exothermic reaction and can be utilized to supply all or a large portion of the energy required for the endothermic steam-coal reactions, and greatly mitigate the need for oxygen, and hence greatly mitigates the need for supporting air separation units in gasification operations. In addition, since higher temperatures means faster steam-gasification kinetics, then higher reactor temperatures imply smaller reactor sizes per unit of syngas exiting the reactor. The trade-off is between higher kinetics for steam-gasification and eventual steam-reforming of the methane. The optimal temperature of the reactor is between 700° C. and 900° C.
It is also understood that the reactivity of carbonaceous materials such as graphite and coal char towards CO2 and steam is strongly enhanced by the presence of alkali metal salts such as Li2CO3, Na2CO3, and K2CO3. See e.g., Sheth et al., “Catalytic gasification of coal using eutectic salts: reaction kinetics with binary and ternary eutectic catalysts,” Fuel 82 (2003), among others. The exact role that the salts play in these processes is not completely understood, however whatever the detailed mechanism of the catalytic process is, the overall rate of gasification is enhanced through contact between the alkali metal catalyst and the carbon. Generally, molten catalyst salts are better able to penetrate the coal structure and, hence, improve accessibility of the unavailable carbon sites in the interior of the coal/char. Similarly, the alkali hydroxide catalysts KOH and NaOH have been utilized in catalytic gasifications. See e.g., U.S. Pat. No. 3,786,138 to Shalit et al.; see also Kamo et al., “Production of hydrogen by steam gasification of dehydrochlorinated poly(vinyl chloride) or activated carbon in the presence of various alkali compounds,” J Mater Cycles Waste Manag 8 (2006). The hydroxide catalysts are generally used in stoichiometric excess to CO2 generated during the gasification process, in order to absorb CO2 and drive the water-gas shift reaction in the gasifier toward exclusive H2 production, and additionally utilize relatively large amounts of H2O for the additional production of H2. These molar relations fail to realize production of a methane-rich syngas at the typical SOFC temperatures of 700-900° C. It would be advantageous to provide a gasification process whereby catalytic gasification using an alkali metal catalyst could generate a methane-rich syngas within the SOFC operating temperature range. It would provide further advantage if the catalyst were an alkali hydroxide, so that a substantial amount of CO2 generated as a result of the gasifier process could be captured within the reactor. The exothermic capture reactions, along with the exothermic methanation reactions, mean that the gasifier can be operated without any input of oxygen or external heating. Additionally, by eliminating the requirement for oxygen to maintain the temperature of the reactor, a larger amount of methane can be generated in the gasifier.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.